NCERT Revision Notes for Chapter 6 Physical and Chemical Changes Class 7 Science
CBSE NCERT Revision Notes1
Answer
- Types of changes -Physical and Chemical
- Difference between Physical and Chemical Change
- Few Examples of Chemical Changes:
- Protection of Rusting
2
Answer
Physical Change
→ Properties such as shape, size, colour and state of a substance are called its physical properties.
→ A physical change in which a substance undergoes a change in its physical properties is called a physical change.
Chemical Change
→ A change in which one or more new substances are formed is called a chemical change.
→ A chemical change in which a substance undergoes a change in its chemical properties is called a chemical change.
3
Answer
Physical Change | Chemical Change |
1. The chemical composition of a substance does not change. | 1. The chemical composition of a substance changes. |
2. Most changes are reversible. | 2. Most changes are irreversible. |
3. No new substances are formed. | 3. No new substances are formed. |
For example: Ice → Water → Steam | For example, Paper → Ashes |
4
Answer
• Burning of Magnesium Ribbon
→ Magnesium ribbon burns with a brilliant white and leaves behind a powdery ash when completely burnt.
Magnesium (Mg) + Oxygen (O2) → Magnesium oxide (MgO)
Dissolving magnesium ash in water
Magnesium oxide (MgO) + Water (H2O) → Magnesium hydroxide [Mg(OH)2]
*Note:- Melting of wax is Physical as well as Chemical Change.
Copper Sulphate Chemical Reaction
→ When copper sulphate reacts with iron, the colour of the solution changes from blue to green due to the formation of iron sulphate.
• Copper Sulphate solution (blue) + Iron → Iron Sulphate solution (green) + Copper (brown deposit)
Crystallization
Crystallization is an example of physical change. The process of crystallization is used for
purification of some substances.
Rusting
→ Rusting is an example of chemical change.
Iron (Fe) + Oxygen (O2, from the air) + water (H2O) → rust (iron oxide Fe2O3)
→ Presence of both air and water is essential for rusting to take place.
→ Rusting can be prevented by cutting the contact of either air or water or both with iron.
5
Answer
→ The same can be done by greasing, oiling, painting, and galvanizing iron.
→ The process of depositing a layer of zinc on iron is called galvanization.
→ Rusting can also be checked by alloying iron with other elements. Stainless steel is an alloy of iron with carbon, chromium, nickel, and manganese.